Production of fluid fertilizer from phosphorus furnace waste stream

ABSTRACT

Processes and compositions of matter are disclosed for the production of liquid fertilizers wherein wastewater from a phosphorus smelting furnace is incorporated in liquid fertilizer processes. The wastewater replaces water evaporated and the wastewater dissolves fertilizer salts. A serious water pollution problem is avoided when wastewater is incorporated in liquid fertilizers. The invention discloses a process for making orthophosphate suspension fertilizer wherein impure phosphoric acid is neutralized in the condensing system, water from the condensing system is bled off, and a suspending clay is added to produce orthophosphate suspension fertilizer. In this process, phosphorus sludge made at phosphorus furnaces is used to produce suspension fertilizer, and wastewater from phosphate smelting furnaces is recovered. New compositions of matter are disclosed. A process is disclosed for making phosphoric acid with low impurities content wherein phosphorus sludge is burned to make impure orthophosphoric acid and the impure acid is recycled to an agglomerating step in a process for making elemental phosphorus.

This application is a division of application Ser. No. 428,840, filedSept. 20, 1983, now U.S. Pat. No. 4,451,277 which is acontinuation-in-part of patent application Ser. No. 301,378 filed Sept.11, 1981, entitled PRODUCTION OF FLUID FERTILIZER FROM PHOSPHORUSFURNACE WASTE STREAM, now U.S. Pat. No. 4,383,847, which is a divisionof application Ser. No. 223,122 filed Jan. 7, 1981, entitled ENERGYCONSERVATION AND POLLUTION ABATEMENT AT PHOSPHORUS FURNACES, now U.S.Pat. No. 4,372,929.

The technical information in this application supports the following twoobjectives.

Provide new processes for making liquid fertilizers whereinphosphorus-containing liquid wastes are incorporated in fertilizermixtures.

Produce electric furnace phosphoric acid of high purity.

Patent application Ser. No. 301,378 contains much background informationon energy conservation and waste recovery at phosphorus smeltingfurnaces used to produce elemental phosphorus. A liquid waste isdischarged at the smelting furnaces, and the waste contains elementalphosphorus. Serious water pollution problems result unless the waste isgiven extensive treatment to remove the elemental phosphorus. Thepresent application focuses on use of the phosphorus-containing waste tomake liquid fertilizer. The present invention provides new processes forusing phosphorus sludge produced at phosphorus smelting furnaces. Thisproblem is closely related to disposal of wastewater containingelemental phosphorus.

Elemental phosphorus is produced by reacting a mixture of phosphate ore,silica rock, and reducing carbon in a smelting furnace called asubmerged-arc electric furnace. The carbon combines with oxygen in thephosphate mineral releasing gaseous phosphorus. Many chemical reactionsoccur in the smelting furnace, and the technology is complex. Thepresent patent application primarily involves recovery of elementalphosphorus from the furnace gas. Nevertheless, a full disclosure of theinvention will require some explanation of the operation and chemistryof phosphate smelting.

The mineral in phosphate ore is fluorapatite and it is represented bythe formula, Ca₁₀ (PO₄)₆ F₂. However, nearly all phosphate ores containfluorapatite in a modified form wherein carbonate and fluorine replacesome of the phosphate radical and some of the calcium is replaced byother metals. Most commercial phosphates are mineral concentrates whichcontain 80 to 90 percent apatite and 10 to 20 percent gangue material.The mineral concentrates are prepared by beneficiating raw phosphateore. Unbeneficiated ores are sometimes used in phosphate smeltingfurnaces and the percentages of apatite will be less than 80 to 90percent because the raw ore is contaminated with clay and silica.

The unsubstituted fluorapatite mineral contains 3.77 percent fluorineand 42.2 percent P₂ O₅ ; its F to P₂ O₅ weight ratio is 0.089. The F toP₂ O₅ weight ratios in commercial phosphate ores may be as high as 0.14.The phosphate ores found in Tennessee and Florida are not highlysubstituted and they have a F to P₂ O₅ weight ratio of 0.106.Unsubstituted fluorapatite must be heated to temperatures well above2000° F. before significant fluorine is volatilized. No significantquantity of fluorine would be volatilized from a phosphorus smeltingfurnace if unsubstituted fluorapatite is the phosphate mineral in theore. However, specific data are not available because unsubstitutedfluorapatite does not occur in commercial phosphate ores. Fluorine insubstituted fluorapatite begins to volatilize at temperatures well below2000° F.

The data on the operation of phosphate smelting furnaces are inadequateto permit precise calculations of the quantity of fluorine volatilized.Data from one test have shown that 9 percent of the fluorine isvolatilized from the smelting furnace when the F to P₂ O₅ ratio in thephosphate was 0.08. Before the phosphate ore was calcined the F to P₂ O₅weight ratio was about 0.11; about 23 percent of fluorine wasvolatilized by calcining at about 2650° F.

Operating experience has shopwn that volatilization of fluorine fromphosphate smelting furnaces increases with the F to P₂ O₅ ratio in theore. The volatilized fluorine causes operating problems in thephosphorus condensing system because insoluble fluosilicate salts areformed in condenser water. In the present invention processes aredescribed to correct the condensing system operating problems whenphosphate ores are smelted having a range of F to P₂ O₅ weight ratiosfrom 0.08 to 0.138.

Silica rock provides SiO₂ to combine with the CaO in phosphate ore tomake calcium silicate slag. Phosphate ores contain some Al₂ O₃, and SiO₂combines with the Al₂ O₃ to form calcium aluminate slag. The slag flowsfrom the furnace as a high temperature liquid. About 18.8 percent of theelectric energy input is lost in the molten slag, and this energy lossamounts to 7.7 million Btu per ton of phosphorus produced. Energy lossis greater for unbeneficiated phosphate ores than for beneficiatedphosphate ores; the unbeneficiated ores contain gangue material whichincreases the quality of slag produced. The percent P₂ O₅ in thephosphate-silica rock mixture is a measure of the energy loss in themolten slag. Barber and Marks in Journal of Metals, December 1962,reported that a decrease of 1 percent in the P₂ O₅ content of thephosphate plus silica mixture increases the electrical energyconsumption about 2.5 percent. In the present invention phosphatesmelting processes are disclosed wherein the P₂ O₅ content of thephosphate plus silica rock mixture is increased to decrease the electricenergy requirement for smelting.

Various condenser system arrangements are used to recover elementalphosphorus from furnace gas. The following two problems are common atall the production units.

Part of the condensed phosphorus is recovered as an emulsion comprisedof water, solids, and discrete particles of elemental phosphorus.Recovery of the elemental phosphorus from the emulsion is costly, may behazardous, and air and water pollution may be made worse by the recoveryprocess.

Water inevitably comes in contact with elemental phosphorus. When thisoccurs, water becomes saturated with the element at a concentration ofabout 0.03 ppm. Also, phosphorus becomes suspended in the water ascolloidal particles. Neither dissolved phosphorus nor colloidalsuspended particles are easily removed from the water. Serious waterpollution problems are caused by the discharge of the contaminatedwater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a Phosphorus Condensing System.

FIG. 2 is a graph of solubility of Na₂ SiF₆ in water and phosphoric acidsolution.

FIG. 3 is a graph similar to FIG. 2 of K₂ SiF₆.

FIG. 4 is a flow diagram showing a process for making phosphoric acid.

FIG. 5 is a flow diagram showing a process for making orthophosphatesuspension fertilizer.

Furnace gas is shown as 1 in FIG. 1. The gas is anhydrous and itcontains about 24.9 pounds of elemental phosphorus per 1000 cubic feetof noncondensable gas at STP. The gas mixture is conducted through line2 to electrostatic precipitator 3. Dust is discharged at 4 at a rate of0.07 ton per ton of elemental phosphorus produced. Treated gas from theprecipitator flows through pipe 5 to the bottom of an adiabaticcondenser 6. Gas is saturated with water by spraying water into the gasby means of water spray 7. Gas is cooled adiabatically from 600° to 145°F. Mixture of water and liquid phosphorus flows down adiabatic condenser6 to collecting sump 8 wherein liquid phosphorus 9 collects as a bottomlayer; an emulsion of solid particles, water, and phosphorus dropletscollect as layer 10; and water 11 is top layer in sump 8. Boundarybetween water 11 and phosphorus sludge 10 is not clearly defined. Gasleaves the adiabatic condenser through pipe 12 and enters tubularcondenser 13. Water overflows a baffle in sump 8. Baffle is not shown inFIG. 1. Water is recirculated from sump 8 to condenser 6 through pipe14. Pump 22 pumps water to spray 7. A stream of the recirculating waterenters tubular condenser 13 through pipe 15 and is sprayed into tubes byspray nozzles not shown. A stream of the recirculating water is bled offat 16 and makeup water is added at 17. The concentration of fluosilicatesalts in the recirculating water is kept below saturation by adding thefresh water. Gas is exhausted by liquid piston pump 18 using as liquidwater which is recirculated to the pump through line 19. Gas flows toentrainment separator 20 and the composition of the gas 21 is asfollows.

    ______________________________________                                                      Percent by volume                                                             Dry  Wet                                                        ______________________________________                                        CO.sub.2        0.7    0.7                                                    O.sub.2         0.1    0.1                                                    CO              87.3   85.9                                                   H.sub.2         7.9    7.7                                                    N.sub.2         2.3    2.3                                                    CH.sub.4        1.1    1.1                                                    C.sub.2 H.sub.2 0.6    0.6                                                    H.sub.2 O       --     1.6                                                                    100.0  100.0                                                  ______________________________________                                    

Pumps 23 transport phosphorus and phosphorus sludge to storage tanksthrough pipelines not shown.

Air leaks into the furnace gas through feed chutes and openings in thefurnace roof and gas offtake. Some of the phosphorus is oxidized by theoxygen in the inleakage forming P₂ O₅. Some of the P₂ O₅ is collected inthe electrostatic precipitator; the remaining P₂ O₅ is entrained in thegas stream and is carried into the adiabatic condenser. The P₂ O₅combines with water forming phosphoric acid.

Fluorine volatilizes from the phosphorus furnace as SiF₄ and thiscompound combines with water in the adiabatic condenser formingfluosilicic acid, H₂ SiF₆.

Both phosphoric and fluosilicic acids corrode steel. It is commonpractice to neutralize the recirculating water at the adiabaticcondenser with an alkali to control corrosion. The recirculating wateris normally neutralized to a pH in the range of 5.5 to 6.0. When eitherNaOH or Na₂ CO₃ is used to neutralize the water, sodium fluosilicate,Na₂ SiF₆, is formed. When the concentration of Na₂ SiF₆ reachessaturation the salt precipitates as a tenacious scale which accumulatesin spray nozzles and pumps. Consequently, water is bled from therecirculating stream at the adiabatic condenser and fresh water is addedto keep the concentration of Na₂ SiF₆ below saturation.

FIG. 2 gives the solubility of Na₂ SiF₆ in water and in a 15.4 percentphosphoric acid solution. Concentrations must be kept below the valuesshown to prevent serious operating problems from scaling of Na₂ SiF₆.Note that Na₂ SiF₆ is more soluble in phosphoric acid than in water, andhigher concentrations of Na₂ SiF₆ can be tolerated in the unneutralizedcondenser water. However, the condensing system must be constructed ofcorrosion-resistant materials if the water is unneutralized.

Sodium and potassium compounds are present in phosphate ores. Bothsodium and potassium substitute for calcium in the apatite molecule.These alkali metals are constituents of clay, a substance frequentlyused as a binder for the agglomeration of phosphate ores. Both sodiumand potassium volatilize from the phosphorus furnace and enter thecondensing system. Much of the potassium is removed from the gas streamby the electrostatic precipitator; the precipitator dust contains about16 percent K₂ O. Nevertheless, some potassium enters the adiabaticcondenser and combines with H₂ SiF₆ forming K₂ SiF₆. The solubility ofK₂ SiF₆ in water and in 15.4 percent phosphoric acid solution is shownin FIG. 3. Note that K₂ SiF₆ is much less soluble in both water andphosphoric acid than is Na₂ SiF₆.

Ammonia is frequently used to neutralize condenser water, and ammoniumfluosilicate will be formed. This salt is more soluble than eithersodium or potassium fluosilicate. Problems of scale formation from theprecipitation of ammonium fluosilicate are rare. However, water must bebled from the recirculating stream to prevent scaling by Na₂ SiF₆ and K₂SiF₆. Electrostatic precipitators are not completely effective for theremoval of particulates from the furnace gas. The gas entering theadiabatic condenser contains particulates which become suspended in therecirculating water. The suspended particulate concentration willincrease to the point that the recirculating water will become slimy andthe water spray will not effectively cool the gas by the adiabaticprocess. Water must be bled from the recirculating stream to preventsuch high concentrations of suspended solids. The bleedoff rate must beincreased if the phosphate ore contains clay: more sodium and potassiumfluosilicate salts accumulate in the condenser water; clay containsmicron size particles which become suspended in the furnace gas; andsolids accumulate in the condenser water at a greater rate.

Water bled from the condensing system contains nutrients, fluorine, andelemental phosphorus as pollutants. The water is saturated withelemental phosphorus at a concentration of about 0.03 ppm. Also, theelemental phosphorus is present as colloidal particles suspended in thewater as described by Isom in Journal of Water Pollution ControlFederation, volume 32, No. 12, pages 1312-16 (1960). The elementalphosphorus concentration should be reduced to the detection limit foranalysis, or about 0.5 ppb, if the effluent is discharged as a waste.Few treatment methods are capable of reaching such low concentrations,and treatment is costly. In patent application Ser. No. 301,378, Idisclosed processes wherein the bleedoff water may be incorporated inorthophosphate fertilizer suspensions without costly treatment. Thepresent invention discloses additional processes for incorporating thewastewater in fluid fertilizer processes.

The prior art reveals several processes for treating phosphorus sludgeto recover high quality phosphorus. In most of the processes, phosphorusis only partially separated from the solid impurities, or fuel must beburned to volatilize elemental phosphorus and water leaving aphosphorus-free residue. Elemental phosphorus is recovered by condensingthe volatilized gases. Some silicon tetrafluoride, SiF₄, volatilizesfrom the phosphorus sludge and contaminates the condensed phosphorus.All of the processes are too costly.

A process is disclosed in the present invention wherein phosphoric acidwith low impurities content is produced from phosphorus sludge. FIG. 4is a diagram which illustrates the process. Phosphate ore 31 and groundphosphate 32 are added to agglomerator 33. Impure orthophosphoric acidis added through line 46. Agglomerates 34 are anhydrous after beingindurated at a temperature of 220° to 1500° F. in 33. The agglomeratedphosphate ore is smelted in 37. Reducing carbon 35 and silica rock 36are added. Phosphorus furnace gas 38 flows to a condensing system 39wherein a mixture of phosphorus and phosphorus sludge 40 is recoveredand pumped to a vessel 11 for settling. High quality phosphorus isrecovered by settling in 41 and is pumped through line 42 to acid unit44 wherein 47 is phosphoric acid product with low impurities content.Phosphorus sludge separated in 41 is pumped through line 43 to acid unit45 wherein orthophosphoric acid with high impurities content is recycledthrough line 46 to the agglomerator 33.

When phosphate ores are smelted in an electric furnace, dust particlesare entrained by the gases generated in the furnace. Also, someinorganic compounds are volatilized from the furnace. At some plantselectrostatic precipitators are provided to remove particulates from thegas stream. However, the precipitators are not completely effective andsome of the particulates remain suspended in the gas stream aftertreatment. The electrostatic precipitators are not designed to removevolatilized inorganic compounds, although some of these compoundscondense in the precipitators and cause operating problems. Particulatesand volatilized inorganic compounds are removed from the gas stream inthe condensing system. The particulates collect with phosphorusparticles and water to form an emulsion having a specific gravity rangeof about 1.3 to 1.5. Liquid elemental phosphorus has a specific gravityof 1.7. Phosphorus sludge is readily separated from liquid phosphorus bysettling, and the liquid phosphorus layer is high purity.

Phosphoric acid is normally produced by burning the material in astainless steel combustion chamber. A coating of polymerized phosphoricacid condenses on the inside surface of the combustion chamber andprevents the stainless steel from corroding. The phosphorus burned mustbe nearly anhydrous to prevent the phosphoric acid coating fromhydrolyzing to orthophosphoric acid. Humidity in air and the smallamount of water entrained and dissolved in good quality phosphorusprovide enough water to form polymerized phosphoric acid needed toprotect the stainless steel combustion chamber. Phosphorus sludgecontains too much water to burn in a stainless steel combustion chamber.

In FIG. 4, item 44 is an acid unit described in the publication,Industrial and Engineering Chemistry, volume 59, No. 6, June 1967, pages18-28, "A High Temperature Superphosphoric Acid Plant," H. Y. Allgood,F. E. Lancaster, Jr., J. A. McCollum, and J. P. Simpson. The combustionchamber is constructed of A.I.S.I. Type 316 stainless steel. Highquality phosphorus must be burned in the unit to prevent corrosion ofthe stainless steel.

The acid unit indicated 45 in FIG. 4 was described in the publication,Corrosion, volume 14, August 1958, "Corrosion Problems in theManufacture of Phosphoric Acid from Elemental Phosphorus," J. C. Barber.The combustion chamber is constructed of graphite blocks. High qualityphosphorus or phosphorus sludge may be burned in the acid unit.

When the acid is recycled to the agglomerator, as disclosed in thepresent invention, impurities are refined in the smelting furnace andall of the product is clear phosphoric acid with low impurities content.All of the product may be highly concentrated phosphoric acid, if thisproduct is desired.

FIG. 5 is a diagram which discloses a process for making suspensionfertilizer from the impure orthophosphoric acid. A mixture of phosphateore, reducing carbon, and silica in 51 is smelted in furnace 52. Furnacegas 53 is treated in condensing system 54, and mixture of phosphorus andphosphorus sludge 55 is transported to settling tanks 56. Material ispumped from tanks 56 through line 57 with phosphorus sludge transportedthrough line 58 to acid unit 60. Phosphorus sludge is burned in acombustion chamber constructed of graphite blocks, and orthophosphoricacid is produced which is recycled through line 62 to the condensingsystem 54 wherein acid is neutralized by ammonia from a line not shownin the diagram. Water containing ammonium phosphate in a saturatedsolution and as crystallized suspension is transported through line 63to a tank 64 equipped with an agitator not shown. A suspending clay isadded to the tank by a feeder not shown in the diagram. Orthophosphatesuspension fertilizer is produced and is pumped from the tank throughline 66. High quality phosphorus from settling tanks 56 is transportedthrough line 59 to acid unit 61. The acid unit is equipped with astainless steel combustion chamber. Phosphoric acid with low impuritiescontent is removed through line 65.

A process is disclosed in FIG. 5 wherein both phosphorus sludge andexcess condenser water are used to make suspension fertilizer. Air usedto burn the phosphorus sludge in acid unit 10 is adjusted so as toproduce an acid mixture containing P₂ O₅ and P₂ O₃. Neutralization ofthe acid mixture with ammonia results in the formation of small sizedammonium phosphate crystals suitable for use in making suspensionfertilizers. Heat generated by neutralization of orthophosphoric acidwith ammonia is removed by a tubular cooler in the condensing system.

In order that those skilled in the art may better understand how thepresent invention can be practiced, the following examples are given byway of illustration but not necessarily by way of limitation.

EXAMPLE I

Phosphate ore was mined in middle Tennessee and was beneficiated bywashing, an operation which removed most of the clay. The beneficiatedore had the following dry basis analysis.

    ______________________________________                                        Constituent      Percent                                                      ______________________________________                                        P.sub.2 O.sub.5  25.9                                                         CaO              36.8                                                         SiO.sub.2        22.7                                                         Fe.sub.2 O.sub.3 3.1                                                          Al.sub.2 O.sub.3 4.0                                                          F                2.8                                                          K.sub.2 O        0.8                                                          MgO              0.3                                                          MnO.sub.2        0.3                                                          Na.sub.2 O       0.6                                                          CO.sub.2         1.5                                                          SO.sub.2         1.7                                                          Organic C        0.2                                                          Combined water   1.5                                                          ______________________________________                                    

The F/P₂ O₅ ratio in the ore was 0.108 as compared with 0.089 influorapatite. Analysis indicates moderate substitution of fluorine forphosphate in the fluorapatite molecule. The phosphate ore wasagglomerated by nodulizing at a temperature of 2650° F. All of the CO₂,SO₂, organic C, and combined water was volatilized; 25 percent of thefluorine came off. As a result, the P₂ O₅ content of the phosphate wasincreased by a factor of 1.06, and the nodulized phosphate had thefollowing composition.

    ______________________________________                                               Constituent                                                                           Percent                                                        ______________________________________                                               P.sub.2 O.sub.5                                                                       27.5                                                                  CaO     39.0                                                                  SiO.sub.2                                                                             24.1                                                                  Fe.sub.2 O.sub.3                                                                      3.3                                                                   Al.sub.2 O.sub.3                                                                      4.2                                                                   F       2.23                                                                  K.sub.2 O                                                                             0.8                                                                   MgO     0.3                                                                   MnO.sub.2                                                                             0.3                                                                   Na.sub.2 O                                                                            0.6                                                            ______________________________________                                    

The F/P₂ O₅ ratio was 0.081 indicating that all of the substitutedfluorine and about 9 percent of the fluorapatite fluorine had beenvolatilized. Nodules, silica rock, and metallurgical coke were chargedto a smelting furnace operating at a powerload of 13,400 kW. Furnace gaswas treated in an electrostatic precipitator to remove suspendedparticulates and the gas was cooled in an adiabatic condenser by astream of recirculating water. A 4.4 percent soda ash solution was addedto the recirculating water to maintain the pH at 5.5 and preventcorrosion. The rate of soda ash addition was 3 pounds per ton ofelemental phosphorus produced. A stream of water was bled off thecondensing system and fresh water was added to control the concentrationof suspended solids in the recirculating stream. Water was bled at arate of 73 pounds per ton of phosphorus produced. Fluosilicates did notprecipitate in the condensing system. A fluorine balance showed that91.4 percent of the fluorine charged was in the slag and 9.1 percent wasin phosphorus sludge, making 100.5 percent accounted for. The water bledoff contained about 0.16 percent fluorine and about 1.7 percent P₂ O₅.Losses of F and P₂ O₅ in the water were 0.1 pound of F per ton ofelemental phosphorus and 1.2 pounds P₂ O₅ per ton. The water containedabout 0.4 percent suspended solids.

The water bled off was added to the first-stage ammoniator of athree-stage process for the production of orthophosphate suspensionfertilizer to replace water normally used to make the fertilizer.Suspension fertilizer was made at a rate of 392 pounds per ton ofelemental phosphorus produced.

EXAMPLE II

Phosphate ore was mined in middle Tennessee and was beneficiated bywashing as in example I. A mixture was prepared which comprised twoparts beneficiated ore to one part unbeneficiated ore, and the mixturecontained a substantial amount of clay. The mixture was briquettedwherein clay was the binder. The briquets were calcined in a nodulizingkiln as in Example I but the temperature was in the range of 1865° to2200° F. In this temperature range, fluorapatite crystals begin to formand agglomerates become indurated by crystal interlocking. Six percentof the fluorine was volatilized during calcination. Chemical analysis ofthe calcined briquets was as follows.

    ______________________________________                                               Constituent                                                                           Percent                                                        ______________________________________                                               P.sub.2 O.sub.5                                                                       26.6                                                                  CaO     37.5                                                                  SiO.sub.2                                                                             20.0                                                                  Fe.sub.2 O.sub.3                                                                      4.1                                                                   Al.sub.2 O.sub.3                                                                      6.6                                                                   F       2.6                                                            ______________________________________                                    

The F/P₂ O₅ weight ratio in the calcined briquets was 0.098, indicatingabout half of the substituted fluorine had been volatilized.

The calcined briquets, silica rock, and metallurgical coke were chargedto a smelting furnace operating at a power-load of 6580 kW. Furnace gaswas treated in an electrostatic precipitator to remove suspendedparticulates and the gas was cooled in an adiabatic condenser. Thecondenser water was neutralized with soda ash solution; 6 pounds of sodaash was required per ton of elemental phosphorus produced. Water wasbled from the condensing system at a rate of 1900 pounds per ton ofelemental phosphorus recovered. The water bled off contained about 0.28percent fluorine. The water was added to the first-stage ammoniator of athree-stage process for the production of orthophosphate suspensionfertilizer to replace fresh water. The suspension fertilizer was made ata rate of 10,800 pounds per ton of phosphorus produced.

EXAMPLE III

A mixture of Florida hard rock phosphate, silica rock, and metallurgicalcoke was smelted in a phosphorus furnace operating at 8050 to 8390 kWpowerload. The Florida hard rock was in lump form and required noagglomeration for use in the phosphorus furnace. Analysis of thematerial charged to the furnace was as follows.

    ______________________________________                                               Constituent                                                                           Percent                                                        ______________________________________                                               Moisture                                                                              0.7                                                                   P.sub.2 O.sub.5                                                                       35.7                                                                  CaO     51.5                                                                  SiO.sub.2                                                                             2.6                                                                   Fe.sub.2 O.sub.3                                                                      1.1                                                                   Al.sub.2 O.sub.3                                                                      0.8                                                                   CO.sub.2                                                                              3.2                                                                   F       3.9                                                            ______________________________________                                    

The F/P₂ O₅ ratio was 0.109 indicating moderate substitution in thefluorapatite. The rate of fluorine volatilization during smelting wascalculated to be 0.07 ton per ton of phosphorus produced. The furnacegas was treated in an electrostatic precipitator to remove suspendedparticulates and the gas was cooled in an adiabatic condenser.Precipitator operation with Florida hard rock was poor because depositsaccumulated on the wires and frames, and these deposits were not removedby steaming or shaking the wires and frame. The deposits causedelectrical grounding of the precipitator. After about a month'soperation it was necessary to shut down and remove the depositsmanually, at which time it was found that crystallized projections onthe wires and frames caused the precipitator grounding. Composition ofthe deposits was as follows.

    ______________________________________                                                    Analysis, %                                                                     Deposits on                                                                              Deposit on                                                         wires in sec-                                                                            second pass                                          Constituent   ond pass   frame                                                ______________________________________                                        Ignition loss 9.6        20.4                                                 P.sub.2 O.sub.5                                                                             35.1       44.0                                                 CaO           0.2        0.2                                                  SiO.sub.2     50.1       32.2                                                 F             0.3        0.3                                                  K.sub.2 O     0.3        0.8                                                  Fe.sub.2 O.sub.3                                                                            0.2        4.2                                                  Al.sub.2 O.sub.3                                                                            0.0        0.0                                                  ______________________________________                                    

The crystalline material deposited in the electrostatic precipitator wasnot identified. It was anticipated that sodium fluosilicate scale wouldprecipitate from condenser water because the F/P₂ O₅ ratio in thephosphate exceeded the ratio for fluorapatite. Consequently, awater-cooled coil was inserted in the condenser sump and the coil wasobserved to determine whether or not fluosilicates were precipitating.Makeup water was added on the basis of observations of the coil. Heatedwater was added to the gas exhauster to avoid problems of fluosilicatedeposition in this unit.

The condenser water was neutralized with soda ash solution, and about438 pounds was required per ton of elemental phosphorus produced. Waterwas bled off at a rate of 6500 pounds per ton of phosphorus produced,and the P₂ O₅ content of the water was about 0.2 percent. Suspensionfertilizer was produced at a rate of 37,100 pounds per ton of elementalphosphorus produced.

EXAMPLE IV

No test data were obtained when phosphoric acid solution was usedinstead of a neutralized solution to condense elemental phosphorus.However, data are available to calculate the results that would beobtained.

Sixty-eight percent BPL phosphate ore from Florida was agglomerated by alow-temperature process wherein orthophosphoric acid was used toagglomerate the material; the phosphoric acid was neutralized withground 68 BPL phosphate ore, and the agglomerates were indurated atabout 300° F. The composition of the ore was as follows.

    ______________________________________                                               Constituent                                                                           Percent                                                        ______________________________________                                               P.sub.2 O.sub.5                                                                       31.1                                                                  CaO     46.6                                                                  Fe.sub.2 O.sub.3                                                                      2.59                                                                  SiO.sub.2                                                                             7.3                                                                   F       3.3                                                            ______________________________________                                    

The ratio of F to P₂ O₅ was 0.106, and 16 percent of the fluorine was inthe substituted form. Phosphate ore required to produce a ton ofelemental phosphorus was about 9.5 tons and about 0.050 ton of F wasvolatilized from the furnace per ton of elemental phosphorus produced.The quantity of fluorine volatilized from the furnace was too large forsuccessful operation of an electrostatic precipitator and none was used.The calculated quantity of soda ash needed to neutralize the fluosilicicacid in the condensing system was 92.9 pounds per ton of elementalphosphorus produced, and about 3 pounds of soda ash was needed toneutralize the phosphoric acid, making a total consumption of 95.9pounds of soda ash per ton of phosphorus produced.

From FIG. 2, solubility of sodium fluosilicate is given as 0.12 poundper gallon of solution in water and 0.135 pound per gallon for 15.4percent H₃ PO₄. These are solubilities at 140° F. Therefore, bleedoffwater rate will be 6,690 pounds per ton of phosphorus when the solutionis neutral and 5,430 pounds of water if a 15.4 percent H₃ PO₄ solutionis used in the condensing system. The bleedoff water was added to thefirst-stage ammoniator of a three-stage process for the production oforthophosphate suspension fertilizer. The suspension fertilizer was madeat a rate of 38,200 pounds per ton of phosphorus produced when thecondensing water was neutralized. When an acid solution is used forcondensing, the suspension fertilizer is produced at a rate of 31,000pounds per ton of phosphorus produced.

EXAMPLE V

Computations similar to those in example IV were made in the presentexample. It was assumed that the condenser water was neutralized withKOH to form K₂ SiF₆. Sixty-seven pounds of KOH was required per ton ofphosphorus.

Solubility of K₂ SiF₆ is 0.036 pound per gallon in water and 0.061 poundper gallon in 15.4 percent H₃ PO₄. The water bleedoff rate was 44,700pounds per ton of elemental phosphorus. The bleedoff water was added tothe first-stage ammoniator of a three-stage ammoniation process for theproduction of suspension fertilizer. The quantity of suspensionfertilizer produced was 256,000 pounds per ton of phosphorus produced.

EXAMPLE VI

Computations similar to those in example IV were made in the presentexample. It was assumed that the condenser water was neutralized withsoda ash, as in example IV, and the water bleedoff rate was 6,690 poundsper ton of phosphorus when the condenser water was neutral. The bleedoffrate was 5,430 pounds when a 15.4 percent H₃ PO₄ solution was used.

The bleedoff water was used to make a 10-30-0 base fertilizer suspensionin accordance with the following formulation.

    ______________________________________                                                           Lbs. for 1 ton                                             Ingredients        of base suspension                                         ______________________________________                                        Monoammonium phosphate                                                                           1,154                                                      fertilizer, 11-52-0                                                           Anhydrous ammonia    90                                                       Clay                 30                                                       Water                726                                                      Total              2,000                                                      ______________________________________                                    

When the condenser water was neutralized, 18,400 pounds of the base10-30-0 suspension fertilizer was produced. When the condenser water wasa 15.4 percent phosphoric acid solution, 15,000 pounds of the basesolution was produced.

EXAMPLE VII

Phosphate ore having a F to P₂ O₅ weight ratio of 0.106 and a P₂ O₅content of 31.6 was smelted in a phosphorus furnace. Silica rock wasadded to flux the mixture, and metallurgical coke was the reducingcarbon. The furnace gases were treated in an adiabatically cooledcondenser wherein recirculating water was neutralized to a pH of 5.5 bythe addition of anhydrous ammonia. The ammonia requirement was 30 poundsper ton of elemental phosphorus produced. The water bleedoff rate was930 pounds per ton of elemental phosphorus. The water bled off was usedto make 10-30-0 base suspension fertilizer in accordance with theformulation given in example VI. The quantity of the base suspensionfertilizer made was 2,560 pounds per ton of phosphorus produced.

EXAMPLE VIII

This example is similar to example VII except that phosphate ore havinga F to P₂ O₅ ratio of 0.11 and a P₂ O₅ content of 36.2 percent wassmelted. The ammonia requirement was 44 pounds per ton of phosphorusproduced and the bleedoff water rate was 1,350 pounds per ton. Thisbleedoff water was used to make 10-30-0 base suspension fertilizer at arate of 3,720 pounds per ton of elemental phosphorus.

EXAMPLE IX

A mixture of phosphate ore, silica rock, and low volatile bituminouscoal was smelted in a phosphorus furnace wherein the coal was reducingcarbon. The coal had the following dry basis analysis.

    ______________________________________                                        Fixed carbon    79.4%                                                         Volatile matter 14.6%                                                         Ash             6.0%                                                          ______________________________________                                    

An adiabatic condenser was used to condense the phosphorus in thefurnace gas. The condenser water contained 80 to 125 ppm of phenol. TheF to P₂ O₅ weight ratio in the phosphate ore was about 0.095, condenserwater was neutralized with ammonia, and water was bled off at a raterequired to keep the concentration of ammonium fluosilicate belowsaturation. The bleedoff water was used to make 10-30-0 base suspensionfertilizer wherein the bleedoff water replaced fresh water normallyadded to dissolve monoammonium phosphate.

EXAMPLE X

Furnace gas was treated in a condensing system illustrated in FIG. 1,comprising an electrostatic precipitator, an adiabatically cooledcondenser, a tubular condenser, and exhausters to pump the gas. Waterwas bled from the condensing system to control the concentration ofsuspended solids in the water and to control the concentration offluorine. The bleedoff water was combined with water from phosphorusstorage tank overflow, phosphorus tank washouts, phosphorus tank carwashouts, bleedoff water from phosphorus metering systems, and waterfrom gasline washouts. Water from all these sources contained elementalphosphorus, and the water accumulated at a rate of 10,800 pounds per tonof phosphorus produced; its suspended solids content was 3900 ppm andthe elemental phosphorus content was 1850 ppm. The contaminated waterwas clarified in a gravity-type clarifier wherein 92 percent of thesuspended solids and 93 percent of the elemental phosphorus wererecovered as phosphorus sludge. About 48 pounds of elemental phosphoruswas recovered per ton of elemental phosphorus produced. Clarified watercontained about 130 ppm of elemental phosphorus and about 310 ppm ofsuspended solids. The clarified water was reused in the system, but astream of the clarified water was bled off, mixed with cooling water,further clarified by settling in a 14-acre pond, treated with air tooxidize elemental phosphorus, and discharged as an effluent. Theeffluent contained 0.3 to 0.5 ppm of elemental phosphorus. The stream ofclarified water was bled off at a rate needed to control the fluorineconcentration of the recirculating water at the adiabatic condenserwhich depended on the F to P₂ O₅ ratio in the phosphate ore fed to thesmelting furnace.

EXAMPLE XI

Bleedoff water from a phosphorus condenser was centrifuged in acommercial centrifuge unit. The bleedoff water contained 1105 ppm ofelemental phosphorus and the concentration was reduced to 27 ppm bycentrifuging. Underflow from the centrifuge contained 95.6 percent ofthe elemental phosphorus in the bleedoff water.

EXAMPLE XII

Gravitational settling tests were made to clarify the water. Thirtyparts per million of Purifloc 601, a cationic flocculent, and 1 part permillion of Purifloc 501, an anionic flocculent, was added to the water.The water contained 44 ppm of elemental phosphorus before settling andthe phosphorus content of supernatant liquor was 1 ppm. The supernatantliquor was clear but it was not water white.

EXAMPLE XIII

Phosphate ore was smelted in electric furnaces to produce elementalphosphorus. The furnace gas was cooled and a mixture of water, elementalphosphorus, and solid impurities was collected in sumps under thecondensers. Liquid phosphorus having a higher density than the othermaterials settled on the bottom of the sump. Phosphorus sludge,comprised of particles of phosphorus, particles of solid impurities, andwater, collected as a layer on top of the phosphorus. Condenser watercollected on top of the phosphorus sludge. Liquid phosphorus andphosphorus sludge were pumped daily to an empty storage tank having acapacity of 150 tons of phosphorus. Plant production rate was about 109tons per day of elemental phosphorus. The mixture in the storage tankwas allowed to settle for approximately 24 hours. Levels of thephosphorus layer and phosphorus sludge layer were measured and samplesof the two materials were taken and analyzed. The quantity and qualityof the two materials were determined. Phosphorus was pumped to storageby a submerged pump through a sight glass, and when phosphorus sludgeappeared in the sight glass, pumping was diverted to phosphorus sludgestorage. Measurements showed that 92 percent of the elemental phosphoruswas high quality product containing 99.6 percent phosphorus. Eightpercent was recovered as phosphorus sludge which varied widely incomposition.

EXAMPLE XIV

A test was made at a 15,000-kW furnace equipped with an electrostaticprecipitator and an adiabatic condenser. The purpose of the test was tocompare the quality and quantity of phosphorus sludge with and withoutusing the electrostatic precipitator. For the test without aprecipitator, electric current was turned off but the furnace gascontinued to flow through the precipitator tubes. When the precipitatorwas operating, 12.2 percent of the phosphorus produced was collected asphosphorus sludge, and the sludge contained 35.7 percent phosphorus.Without operation of the precipitator, 27.8 percent of the phosphorusproduced was collected as sludge, but the sludge contained 52.2 percentphosphorus. With the precipitator operating, the phosphorus contained98.8 percent of elemental phosphorus; without the precipitatoroperating, the phosphorus contained 96.4 percent elemental phosphorus.Smelting of phosphate ore with a high F/P₂ O₅ ratio will precludeoperation of electrostatic precipitators because crystallized depositsaccumulate on the frames and wires and cause electric grounding. Largeramounts of phosphorus sludge must be processed if precipitators are notused.

EXAMPLE XV

Phosphorus separated from phosphorus sludge by settling was burned in astainless steel phosphoric acid unit to produce concentrated acid ofhigh purity. Following is an analysis in percent by weight.

    ______________________________________                                                      Percent                                                         Constituent   by weight                                                       ______________________________________                                        P.sub.2 O.sub.5                                                                             79.9                                                            Fe            0.001                                                           Al            0.001                                                           Cr            None found                                                      Ni            0.001                                                           Mo            None found                                                      As            0.001                                                           Pb            0.0002                                                          SiO.sub.2     0.001                                                           CaO           0.004                                                           F             0.01                                                            Cl            None found                                                      S             0.002                                                           ______________________________________                                    

The metal/P₂ O₅ ratio was 95×10⁻⁶, and the F/P₂ O₅ ratio was 125×10⁻⁶.The high quality phosphoric acid can be used in making animal feedsupplements and in preparing other high quality phosphate chemicals.Also, highly concentrated phosphoric acid was produced which had the P₂O₅ distributed among various species of phosphoric acids as shown below.

    ______________________________________                                                     Percent                                                          ______________________________________                                               Ortho   17.5                                                                  Pyro    40.1                                                                  Tripoly 22.1                                                                  Tetrapoly                                                                             11.5                                                                  Pentapoly                                                                             5.6                                                                   Hexapoly                                                                              2.1                                                                   Heptapoly                                                                             1.2                                                            ______________________________________                                    

EXAMPLE XVI

Phosphorus was produced at a smelting furnace wherein 8 percent of theelemental phosphorus was in the phosphorus sludge and 92 percent was inthe high quality phosphorus. The phosphorus sludge was comprised of 80percent elemental phosphorus and 20 percent benzene-insoluble impuritieson a dry basis. The benzene-insoluble impurities had the followingcomposition.

    ______________________________________                                                       Percent                                                        ______________________________________                                        P.sub.2 O.sub.5  32                                                           F                23                                                           CaO              9                                                            SiO.sub.2        7                                                            Fe.sub.2 O.sub.3 6                                                            Al.sub.2 O.sub.3 3                                                            C (by difference)                                                                              20                                                           Total            100                                                          ______________________________________                                    

The phosphorus sludge was burned in a combustion chamber. The combustionchamber was a graphite electrode which had been bored out. The hydratorwas a graphite block chamber, and a stainless steel venturi scrubberremoved particulates from the gases. Phosphoric acids produced in thecombustion chamber, hydrator, and venturi scrubber were combined, andthe concentration of the combined acid was 53 percent P₂ O₅. The acidcontained the following percentage of impurities.

    ______________________________________                                                      Percent                                                         ______________________________________                                        F               0.1                                                           CaO             0.5                                                           SiO.sub.2       0.4                                                           Fe.sub.2 O.sub.3                                                                              0.3                                                           Al.sub.2 O.sub.3                                                                              0.2                                                           C               1.1                                                           Total impurities                                                                              2.6                                                           ______________________________________                                    

The water used to hydrate the P₂ O₅ to form acid was clarified condenserwater. The quantity was 340 pounds per ton of elemental phosphorusproduced.

The impure phosphoric acid produced from phosphorus sludge was used toagglomerate phosphate ore in accordance with the following formulation.

330 parts 68% BPL phosphate ore

42 parts ground phosphate

63 parts impure phosphoric acid produced from phosphorus sludge

85 parts water

Approximately 4,000 pounds of water per ton of phosphorus was requiredto agglomerate the phosphate ore. Clarified condenser water can be usedto provide the agglomerating water. Impure acid produced from phosphorussludge was therefore recycled to the phosphorus furnace.

EXAMPLE XVII

Phosphorus sludge was processed in a phosphoric acid production unitwherein the sludge was injected into a horizontal chamber called avaporizer. Sufficient air was put into the vaporizer to burn about 20percent of the elemental phosphorus. The remainder of the elementalphosphorus and the water were vaporized; they flowed to a verticalcombustion chamber through a duct. Additional air was put in thecombustion chamber to burn the phosphorus. The gases were hydrated bywater sprays in another vertical chamber. The gases were conducted to aventuri scrubber by means of a duct. Phosphoric acid was recovered inthe combustion chamber, hydrator, and venturi scrubber. The combinedacid contained 58 percent P₂ O₅ and 3 percent P₂ O₃.

The phosphoric acid mixture was used to make diammonium phosphatefertilizer by the vacuum crystallization method. Much of the P₂ O₃ inthe phosphoric acid remained dissolved in the mother liquor and theconcentration of P₂ O₃ exceeded 3 percent. Excessive nucleation ofdiammonium phosphate crystals occurred. A crystal slurry was obtainedwhich could not be centrifuged because of the fineness and gelatinouscharacter of the solids. The crystal slurry had properties desired forthe preparation of suspension fertilizers. When the P₂ O₃ concentrationof the mother liquor was in the range of 1.5 to 3 percent and about 8percent of SO₄ ions were present, the diammonium phosphate crystals werelarger and thicker and they had properties desired for diammoniumphosphate as crystal fertilizer.

EXAMPLE XVIII

In this example a process is described for treatingphosphorus-containing waste at a plant which converts elementalphosphorus into other phosphorus chemicals. However, the total processhas not been carried out as described.

Elemental phosphorus was received in tank cars and pumped to storagetanks. A layer of water was kept over the phosphorus to prevent it fromburning. Phosphorus was pumped to metering tanks for processing into thechemicals. Water displaced phosphorus in the metering tanks; water ratewas measured and its rate was controlled in order to measure and controlthe phosphorus feed rate.

When water comes in contact with elemental phosphorus the water becomessaturated with the element, and colloidal particles of phosphorus becomesuspended. The water was reused in a closed system, but fresh makeupwater was added at a rate needed to keep the maximum concentration ofelemental phosphorus at 100 ppm. Water was bled from the system asmake-up water was added. The water bled from the system was treated witha flocculent and clarified by settling. The elemental phosphorus contentof the supernatant water was less than 1 ppm and it was returned to therecirculating water system. Underflow from the settling operation wasused to prepare a 10-30-0 base suspension fertilizer wherein ammonia,solid ammonium phosphate fertilizer, and the underflow were mixed todissolve the monoammonium phosphate. A precipitate formed fromimpurities in the monoammonium phosphate, and a suspending clay wasadded to keep the precipitate in suspension.

EXAMPLE XIX

The integrated process described below has not been carried out, buttechnical data were obtained on most of the steps comprising theprocess.

A mixture of phosphate ore, silica rock, and reducing carbon was smeltedin an electric furnace. The furnace gas was treated in a condensingsystem wherein most of the elemental phosphorus was recovered in anadiabatic condenser. A mixture of liquid phosphorus and phosphorussludge was pumped to a settling tank to separate the two materials. Thephosphorus sludge was converted to orthophosphoric acid in a productionunit comprised of a vaporizer, graphite combustion chamber, carbon blockhydrator, and a stainless steel venturi scrubber. The product was amixture of phosphoric and phosphorous acids; the mixture contained 53percent P₂ O₅ and 3 percent P₂ O₃. In addition, the acid contained 1.5percent inorganic impurities and 1.1 percent carbon. The acid was black;it was added to the condenser water and this water was neutralized to apH of about 6 by ammonia. Small sized ammonium phosphate crystalsprecipitated in the recirculating water and were suspended by theturbulence in the recirculating water. A stream of the recirculatingwater was bled off and pumped to a tank equipped with an agitator. Priorto filling the tank a gelled clay was put in. The mixture was agitatedand pumped to storage for sale as a suspension fertilizer.

Liquid phosphorus separated from the phosphorus sludge was converted tohighly concentrated phosphoric acid with low impurities content. Themetals/P₂ O₅ ratio in the acid was about 70×10⁻⁶, and the F/P₂ O₅ ratiowas about 130×10⁻⁶.

I claim:
 1. A process for the production of phosphoric acid wherein:(a)A mixture of phosphate ore containing fluoroapatite, phosphoric acid,and ground phosphate is agglomerated by tumbling; (b) agglomerates instep (a) are indurated in the temperature range of 220° to 1500° F.; (c)indurated agglomerates in step (b) are mixed with reducing carbon andsilica rock; (d) mixture in step (c) is smelted in a submerged-arcelectric furnace wherein a phosphorus-containing gas is evolved; (e)phosphorus-containing gas in step (d) is cooled in an adiabaticcondenser wherein water is sprayed into the condenser, collected in asump, and recirculated in a stream to the condenser; (f) a stream ofwater is bled off; (g) phosphorus and phosphorus sludge condensed in theadiabatic condenser is separated by gravitational settling; (h)phosphorus sludge in step (g) is burned in a graphite combustion chamberto produce a gas and phosphoric acid (i) gases from combustion chamberin step (h) are contacted with bleedoff water from step (f) in ahydrator constructed of carbon blocks; (j) gases from hydrator in step(i) are treated in a venturi scrubber; (k) phosphoric acid collected incombustion chamber, hydrator, and venturi scrubber is recycled to step(a); (l) liquid phosphorus separated in step (g) is processed intophosphoric acid in a stainless steel production unit.
 2. A process forthe production of phosphoric acid as in claim 1 wherein the F/P₂ O₅weight ratio of the fluorapatite in the phosphate ore in step (a) is inthe range of 0.095 to 0.140.
 3. A process for the production ofphosphoric acid as in claim 1 wherein the graphite combustion chamber instep (h) is made from a solid, cylindrical piece of graphite.
 4. Aprocess for the production of phosphoric acid as in claim 1 wherein thereducing carbon in step (c) is low volatile matter bituminous coal.
 5. Aprocess for the production of phosphoric acid as in claim 1 wherein instep (b) said agglomerates are indurated in the temperature range of220° to 300° F.
 6. The process of claim 1 wherein phosphorus containingwaste water obtained from the storage of phosphorus in water is added tosaid recirculated stream in step (e).